The field of the present invention pertains to energetic polybutadiene-compatible plasticizers for solid propellant binders.
Most of the solid propellant binders in use today are reaction products of hydroxyl terminated polybutadiene prepolymers (HTPBD), such as R45M, (producer: ATOCHEM, North America) with polyisocyanates, such as isophorone diisocyanate (IPDI). The binders are often diluted up to 40% with "inert" plasticizers, such as dioctyladipate (DOA). Inert plasticizers do not carry energetic groups and serve mainly to improve propellant processability and the flexibility at low temperature. The main disadvantage of inert-plasticized PBD-binders is, besides a small loss of performance (impulse), their need of very high levels of oxidizer to effect complete combustion. The correspondingly reduced volume fraction of the binder, which provides the liquid components of the propellant formulation, causes a high viscosity of the propellant slurry, and thus impaired processability and, usually, poor strain capability in the cured propellant. Low binder fractions are particularly prevalent in ammonium nitrate oxidized propellants, because of the lower density of that oxidizer. Here, other binder systems with a lower oxygen demand are used. Typically, polyester and/or polyether prepolymers highly diluted (up to 80%) with "high-energy plasticizers" are employed. The latter are the nitrate esters of glycerol, butanetriol, trimethylolethane, and tri-and diethyleneglycol, or nitro- compounds, such as the formals and/or acetals of 2,2-dinitropropanol, 2,2,2-fluorodinitroethanol, etc. All of these are liquid and belong to the class of "high-explosives". Their principal function is to supply oxygen, at least for their own combustion, while simultaneously providing a low viscosity liquid for improved processability.
Useful, PBD-binder-compatible, energetic plasticizers were neither known nor were they readily available. None of the above mentioned high-energy nitrato- or nitro-compounds is soluble to any significant extent in polybutadiene prepolymers. Even if they were, the disadvantages that they impart, namely a significantly increased detonabilty hazard and a high pressure exponent of the burn rate, would make such systems unattractive for many missions.
Azido-compounds, a type of energetic materials that are now being investigated in the industry, are chemically incompatible with the double bonds of polybutadiene, and therefore can not be employed in such systems. Of the energetic compounds, practically only nitrato-and nitro-derivatives are chemically compatible with polybutadienes.
On the other hand, high-energy-plasticizer-compatible polyesters and polyethers are poorly suited for ammonium nitrate oxidized propellants. Polyesters, because of their hydrolytic instability, which is strongly aggravated by the hygroscopicity of ammonium nitrate, and polyethers, because of their solvating power for ammonium nitrate, which causes very viscous, unprocessable propellant batches. Some energetic prepolymers, like GAP and energetic-group-carrying polyoxetanes could be used with ammonium nitrate. Their disadvantages are a questionable availability in large quantities, and at least with GAP, their notorious unreliability. Their chemical instability, and potential detonability, when plasticized with high-energy nitrate esters, and also inherently high pressure exponents as well as cost make such propellant types unsuitable for large booster rockets.